Yours is a BENDIX TREADLE-VAC SYSTEM common to most cars of that period.
In the 54 MERC SHOP MANUAL, is there a diagnostic sequence offered to isolate problem(s)? What is your manifold vacuum?
It seems a major concern is the MC being an aluminum casting and if the system is not service as scheduled, the brake fluid crystallizing.
Read this -
http://packardinfo.com/xoops/html/mo...=18986&forum=4
Rebuild Kits are available -
https://www.parts123.com/parts123/yb...~~~~~MONTCLAIR
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- WHY IS IT ... 
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... that everything you buy in the grocery store is either wrapped in or contained in plastic but the government doesn't want you to carry it out in a plastic bag?
WONDERING IN W(BY GOD)V ...
DIAGNOSED CDO - (OCD In Correct Alphabetical Order)